1. Field of the Invention
This invention relates to a pyroelectric-type infrared ray receiving element for receiving infrared rays (hereinafter abbreviated as xe2x80x9cIRxe2x80x9d) radiated from an object, and an IR sensor using the IR ray receiving element.
2. Description of the Related Art
A pyroelectric-type IR receiving element is conventionally used to a sensor for sensing heat, temperature, approach of human, or the like. A configuration of the conventional pyroelectric-type IR receiving sensor is shown in FIG. 36.
As can be seen from FIG. 36, two pairs of electrodes 2A and 2B, and 2C and 2D are formed on top and bottom surfaces 1A and 1B of a substrate 1 for facing each other. Both sides of the substrate 1 are fixed to stands 3A and 3B formed on a base member 3 by conductive adhesives 4A and 4B.
The substrate 1 is made of a ferroelectric material such as a ceramics of PbTiO3 or Pb(Ti+Zr)O3, a single crystal of LiTaO3, or a high molecular compound of PVF2. Arrow P designates a direction having the largest pyroelectric coefficient in the material of the substrate 1. In the following description, the direction designated by arrow P will be called xe2x80x9cpolarization directionxe2x80x9d.
The electrodes 2A to 2D are made of an IR absorbent material such as NiCr or the like. The electrodes 2A to 2D are formed by vapor deposition, sputtering or screen printing. The electrodes 2A and 2C on the top surface 1A of the substrate 1 respectively serve as IR sensing portions. The electrodes 2A to 2D respectively have the same rectangular shape. The electrodes 2A to 2D are connected to an external circuit by wires or conductive patterns which are not shown in the figure. Two sets of the electrodes 2A and 2B, and 2C and 2D respectively configure capacitors. An equivalent circuit of the conventional IR receiving element is shown in FIG. 37.
When IR reach to the surfaces of the electrodes 2A and 2C, energy of the IR is converted to thermal energy, and temperature at surfaces of the electrodes 2A and 2C increases. When the temperature of the substrate 1 is varied, pyroelectric charges occur in the substrate 1 owing to variation of spontaneous polarization of the pyroelectric material. The occurrence of the pyroelectric charges in the substrate 1 can be sensed by variation of voltage signals through a resistor R and a field-effect transistor FET in the equivalent circuit shown in FIG. 37. As a result, incidence of the IR into the IR receiving element can be sensed.
When ambient temperature of the substrate 1 is varied, the substrate 1 will be warped as shown in FIG. 38 due to a difference of thermal coefficients between the substrate 1 and the base member 3. The warp of the substrate 1 causes the occurrence of undesired electric charges, since the pyroelectric material of the substrate 1 shows piezoelectric effect, too. When the electric charges locally charged due to the piezoelectric effect of the substrate 1 are discharged, the discharge will be observed as unexpected noise signal called xe2x80x9cpopcorn noisexe2x80x9d.
For reducing the occurrence of the popcorn noise, the inventors have proposed to configure the IR receiving portions as the cantilever fashion described in the publication gazette of Japanese Patent Application Hei 10-2793. Concretely, U-shaped slits are formed in the substrate 1 for surrounding three sides of the electrodes 2A to 2D. By such the configuration, the IR receiving portions are substantially formed on the cantilevers, so that no stress occurs in the IR receiving portion, even when the substrate 1 is partially warped. As a result, the occurrence of the popcorn noise can be reduced.
It, however, is difficult to prevent the occurrence of the popcorn noise completely only by the above-mentioned U-shaped slits. The conventional substrate 1 is formed by a material uniformly polarized as shown in FIG. 4, where the polarization direction in any portion is the same. When the substrate 1 is partially warped due to the difference of the thermal expansion coefficients between the substrate 1 and the base member 3 except the IR receiving portions formed in the cantilever fashion, the undesired electric charges occur due to the piezoelectric effect in the warped portion where no conductive pattern is formed. These undesired electric charges generally disappear by coupling with ions which flow in the circumference of the substrate 1. The undesired electric charges, however, sometimes discharge to neighboring conductive patterns, circuit substrate, or metal housing, so that the popcorn noise is rarely observed.
The inventors have carefully experimented and considered the cause of the occurrence of the popcorn noises, and found that piezoelectric effect of the material of the substrate 1 acts as a trigger instantaneously for discharging the undesired electric charges when the stress due to the difference of the thermal expansion coefficients between the substrate 1 and the base member 3 of the adhesive 4A, 4B is applied to the substrate 1 or when external mechanical vibrations are applied to the substrate 1.
An object of this invention is to provide an IR receiving element in which the popcorn noise hardly occurs, and to provide an IR sensor using the same.
An infrared ray receiving element in accordance with this invention comprises: a substrate made of a pyroelectric material and having at least one cantilever portion surrounded by a slit, in which at least a part of the cantilever portion in the substrate is uniformly polarized in the same direction and the remainder in the substrate includes a portion polarized at random; and at least a pair of electrodes respectively provided on a top surface and a bottom surface of the cantilever portion.
An infrared ray sensor in accordance with this invention comprises an infrared ray receiving element, a base member for holding the infrared ray receiving element, a circuit substrate connected to the infrared ray receiving element for detecting that infrared ray reached to an infrared ray receiving portion of the infrared ray receiving element, a chassis for supporting the infrared ray receiving element, the base member, the circuit substrate, and a cover with an infrared ray transmittable window, wherein the substrate is made of a pyroelectric material and has at least one cantilever portion surrounded by a slit, in which at least a part of the cantilever portion in the substrate is uniformly polarized in the same direction and the remainder in the substrate includes a portion polarized at random; and at least a pair of electrodes are respectively provided on a top surface and a bottom surface of the cantilever portion.
By the above-mentioned configurations, the IR receiving portion is formed on the cantilever portion, so that the IR receiving portion is hardly warped even when the ambient temperature of the substrate is varied. Thus, no electric charge due to the piezoelectric effect of the substrate made of the pyroelectric material is generated in the IR receiving portion. Furthermore, most of the substrate except the IR receiving portion is polarized at random, so that electric charges generated in each minute portion in the substrate by the piezoelectric effect when the substrate is partially warped will be canceled by the random direction of the polarization. Thus, the undesired electric charge is hardly charged in the substrate, and the discharge of the undesired electric charge hardly occurs even when stress due to the difference of the thermal expansion coefficients between the substrate and a base member or an adhesive for supporting the substrate is applied to the substrate or when external mechanical vibrations are applied to the substrate. As a result, popcorn noise is hardly observed.